The present disclosure generally relates to defoaming compositions and methods for preventing or breaking foam or entrainment of gas in oil and gas well treatment fluids.
In the drilling and completion of an oil or gas well, a cement composition is introduced to the well bore for cementing pipe string or casing. In this process, known as “primary cementing”, the cement composition is pumped into the annular space between the walls of the well bore and the casing. The cement composition sets in the annular space, supporting and positioning the casing, and forming a substantially impermeable barrier, or cement sheath, which isolates the well bore from subterranean zones. Cement compositions are also used for remedial operations such as squeeze cementing.
Portland cement systems for well cementing are routinely designed to perform at temperatures ranging from below freezing point in permafrost zones to about 350° C. in thermal recovery and geothermal wells. Well cements also encounter the pressure range from near ambient in shallow wells to more than 30,000 pounds per square inch (psi) in deep wells. In addition, they may also be designed to contend with other field and operating conditions. Chemical additives are often used to modify the behavior and properties of cement systems, ideally allowing successful slurry placement, proper setting, and adequate zonal isolation during the lifetime of the well.
Several cement additives such as retarders, dispersants, fluid loss control additives, and gas migration control additives can cause the slurry to foam during mixing. Excessive slurry foaming can have several undesirable consequences such as loss of hydraulic pressure during pumping due to cavitation in the mixing system. In addition, air entrainment may cause undesired slurry densities at downhole. During slurry mixing, a densitometer or mass flow meter is used to help field operators proportion the ingredients. If air is present in the slurry at the surface, the density of the system “cement+water+air” is measured by the densitometer. Since the air becomes compressed downhole, the true downhole slurry density becomes higher than the measured surface density. Antifoaming or defoaming agents are usually added to the mix water or dry-blended with the cement to prevent such problems. They may also be used for breaking foamed fluids. In such applications, defoamer may be utilized to break the excess foamed fluid returned to surface after well treatment and thus facilitate disposal process. In general, desirable antifoaming or defoaming agents, have the following characteristics to be effective: a) insoluble in the foaming system, and b) lower surface tension than the foaming system. The antifoaming agent functions largely by spreading on the surface of the foam or entering the foam lamella. Because the film formed by the spread of antifoam on the surface of a foaming liquid does not support foam, the foam situation is alleviated.
There are two general classes of defoaming agents commonly used for cementing applications: alkoxylated alcohols and silicones. Silicone defoaming agents, while very effective, are not readily biodegradable and are relatively expensive when compared to other chemistries. As to the alkoxylated alcohols, these materials are generally not very effective.
Accordingly, there is an ongoing need for high performance non-silicone defoaming compositions and methods that have better environmental profile and provide defoaming properties equivalent or higher than silicone based defoaming agents for reducing the entrained air in the cement fluids and for achieving desired density.